Modulation system for television



W. J. POCH MODULATION SYSTEM FOR TELEVISION Filed April 20, 1942.

Gttomeg m l m-i July 9, 1946.

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Patented July 9, v1946 MODULATION SYSTEM ron TELEVISION Waldemar J. Pooh, Moorestown, N. .I., assignor to Radio Corporation of America, a corporation of Delaware Application April 20, 1942, Serial No. 439,643

.5 Claims. (01. 178-'7.2)

. 1 My invention relates to modulation systems and particularly to modulation systems for television transmitters.

In portable television equipment one of the problems is to obtain satisfactory synchronizing signal output from a transmitter with the minv imum weight of equipment. An object of the presentinvention is to provide an improved means for modulating a radio transmitter by picture signals and synchronizing pulses. Another object of the invention is to provide an improved system for modulating a carrier wave by a mixture of large amplitude pulses and smaller amplitude signals.

In one of the preferred embodiments of the invention, as applied to a portable television transmitter in which a high'frequency carrier wave is amplitude modulated both by picture signals and by horizontal and vertical synchronizingpulses, the picture signal modulation is obtained by grid modulation while' the synchronizing pulse modulation is obtained by plate and/or screen grid modulation. A modulation system of this character is described and claimed ina copending application, Ser. No. 436,406, filed March '27,.1942, in the name of Henry N. Kozanowski, and entitled Modulation system of television. In accordance with my invention, I include a unilateral conducting device such as a diode in the plate modulating circuit. The diode provides the necessary impedance in the plate circuit for plate modulation during the occurrence of a pulse but it presents a low impedance for grid modulation during the occurrence of theinterspersed picture signals.

The invention will be better understood from the following description in connection with the accompanying drawing, in which Fig. 1 is a circuit diagram of a television transmitter embodying my invention, and Fig. 2 is a circuit diagram of another embodiment of the invention. In the several figures, corresponding parts are indicated by the same reference characters.

In Fig. 1 the invention is shown applied to ,a television transmitter comprising a power ampliher It having a tuned grid circuit II and a tuned anode or plate circuit 12. The amplifier ill ineludes vacuum' tubes l3 and I, each having a cathode, a control grid, a screen grid, and an anode or plate. 'A carrier wave is applied to the tuned grid circuit H as indicated. The amplifier output may be suppliedirom the tuned anode circuit l2 to an antenna (not shown).

In accordance with my invention the carrier wave is grid modulated by picture signal indicated at 15 which is supplied through a resistor Hi from a pickup tube and amplifier I1, and itls plate modulated by horizontal'and vertical synchronizing pulses supplied from a synchronizing pulse generator l8 over a conductor I9 to an amplifier tube 2| which preferably is D.-C. coupled to an amplifier tube 22. Horizontal synchronizing pulses are indicated at 20. As will be explained below, the synchronizing pulses appear across a. cathode resistor 23 of the amplifier tube 22 and are applied with positive polarity to the screen grids and plates of tubes l3 and It to increase their power output during the occurrence of each synchronizing pulse. In the specific ex ample illustrated, the synchronizing pulses are applied to the control grids of tubes l3 and I along with the picture signal but this is not necessary as all synchronizing signals may be transmitted by the plate modulation.

The feature of amplitude modulating the grid with picture signals and amplitude modulating the plate with synchronizing pulses has two im- First, the peaks of the synthe picture signal increasing in amplitude enough to move the black side of the signal into and possiblybeyond the upper knee of the curve obtained by plotting power amplifier grid voltage against radio frequency output current. It may be noted that since the system shown in Fig. 1

employs A.-C. transmission (there being no D.-C.

insertion) the "black side of the picture signals may sometimes-by moved beyond the upper knee of the curve also by an increase in the picture background component, that is, by the overall picture becoming lighter in tone. Secondly, more synchronizing power can be obtained from given power amplifier tubes since the synchron-' izing pulses may drive the plate voltages momentarily beyond any plate voltage that can be applied to the tubes continuously without damaging them,

The use of plate or screen grid modulation in a system such as the one described in the abovementioned Kozanowski application will degenerate the'grld modulation to a certain extent because of the resistance required in the plate circult of the power amplifier. According to my invention, I avoid this undesirable efl'ect by utilizing a unilateral conductingdevice such as a 'diode 26 so connected that there-is the necessary resistance in the plate circuit during plate modulation but substantially no resistance therein during grid modulation. connected between the anodeol' amplifier tube 22 Diode 2s 1s video frequency bypass condensers, respectively.

A voltagesource such as a battery 3| supplies anode voltage to the tube 22 through a conductor 22 and anode voltage to the tube -2| through a coupling resistor 34. -It also Supplies screen grid voltageto the power amplifier tubes l3 and I4 through resistors 28 and II. The capaci-tor 28 across battery 3| may be provided for bypassing the video and synchronizing pulse fre quencies.

A capacitor 4| connected between the cathode of tube 22 and the cathode of the diode 26 is an important element of. my improved circuit because its voltage is added in series to the voltage of source 3| in response to the occurrence of a synchronizing pulse, and this increased voltage is applied to the screen grids and anodes oi the power amplifier tubes l3 and II. This action will be understood by following through the circuit operation as follows: The synchronizin pulses 4 n will be evident that we diode a a nonconducting during the application of a synchronizing pulse since its cathode is driven beyond the positive potential of the diode plate. There- I tore, the diode 26 provides the necessary'impedance in the power amplifier plate circuit for plate modulation. Between synchronizing pulses. and during grid modulation by the picture signal, however, the diode 28 is conducting whereby the impedance of the power amplifier plate circuit is very low. It follows that any undesirable de-' generative efi'ect during the picture modulation intervals is avoided. In the particular example illustrated, the power amplifier tubes H and H are screen grid tubes but it should be understood that they may be triodes if properly neutralized.

Fig. 2 illustrates an embodiment of the invention in which a triode I is substituted for the are applied 'with, negative polarity to-the grid of 1 the tube 2|. Between pulses, therefore, this grid is at about cathode potential and the potential at the anode of the tube 2| is at about its minimum value. This minimum voltage applied to the control grid of tube 22 biases the tube close to plate current cut-oil since its cathode is above ground potential and positive with respect to the control grid due to plate current fiow (even though a small amount) through the cathode resistor 23.

Thus, between synchronizing pulses the anodecathode impedance of tube 22 is high and the capacitor ll is charged substantially to the voltage of the plate supply 3|. This charge is acquired through a path that may be traced from the positive terminal of battery 3| through resister 21, diodell and a conductor 02 to one side of capacitor-l and from the other side of capacitor 4| through cathode resistor 23 to ground and to the negative terminal of source 2|.

Upon the occurrence of a synchronizing pulse, the anode-cathode impedance of tube 22 is reduced to a comparatively low value whereby the time 22 functions as a switch to connect the lower or negative side of capacitor 4| to the positive terminal of the battery 2|.. Since the capacitor 4| is or large capacity, any charge lost during the application of a synchronizing pulse does not lower the voltage thereacross very much. The result of the above switching action is that there is applied to the cathode of diode 28 and to the. anodes and screen grids of power amplifier tubes l2 and I a voltage that is, in an ideal circuit. double the voltage of source 3|. This increased voltage at the positive'side of capacitor ll is impressed upon the power amplifier anodes through resistor 28 and the inductor of tuned circuit l2. It is impressed upon the power ampli fier screen grids through a coupling capacitor 48 and the resistor 21. Consequently, the power output of'tubes I3 and I4 is increased upon the occurrence of synchronizing .pulses, i. e. the carrier wave is amplitude modulated by these pulses. This momentary additional power is actually obtained from the large capacitor 4| which is starin: energy between pulses.-

cathode resistor 23 of Fig. 1. Synchronizing pulses are applied with negative polarity'to the gridof tube 5| irom a cathode resistor 52 of'the amplifier tube 2|. It will be evident that the operation of this embodiment is the same as previously described, the diflerence being that'in Fig. l the cathode resistor 23 is eilectively in shunt to the battery 3| during the application of a synchronizing pulse and will draw some current whereas in Fig. 2 the'tube 5| has high anodecathode impedance during,this time and draws little, if any, battery current. Because of this, the circuit of Fig. 2 may be preferred when the voltage supply source ll is a battery and it is important to keep the current drain to a minimum.

It should be understood that the invention is I not limited to the particular circuits illustrated.

For example, the D.-O. connection between the tubes 2| and 22 in Fig. 1 may be replaced by A.-C. coupling. On the drawing the values of .certain circuit elements have been indicated. merely by way of example, in ohms, microfarads, and micro-microfarads.

I claim as my invention:

1. In a television transmitter, means for producing a carrier wave, an amplifier connected to amplify said carrier wave, said amplifier having a cathode, a control grid and a third electrode, a source of picture signals and synchronizing signals, means for appwing said picture signals to said grid to amplitude modulate said carrier wave by picture signals, and meansfor applying said synchronizing-signals to said third electrode to amplitude modulate said carrier wave by synchronizing signals, said last means including a unilateral conducting device and means for so connecting it as'to provide'external impedance for said third electrode in response to and for the duration of each synchronizing signal and to provide a comparatively small external impedance therefor during the occurrence of picture signals.

. 2. In a television transmitter, means for producing a carrier wave, an amplifier connected to amplify said carrier wave, said amplifier having 'ing signals to said plate to amplitude modulate said carrier wave by synchronizing signals, said last means including a unilateral conducting device and means for so connecting it as to provide external plate impedance for said amplifier in response to and for the duration of each synchronizing signal and to provide a comparativediode and means for so connecting it as to provide external impedance for said third electrode in response to and for the duration of each synchronizing signal and to provide a comparativea direction to conduct current to said electrode, a capacitor and switching means connected in series with each other, said series combination being connected across said device.

5. In a television transmitter, means for producing a carrier wave, an amplifier connected to amplify said carrier wave, said amplifier having a a cathode, a control grid and a third electrode,

' a source of picture signals and synchronizing sigly small external impedance therefor during the -a cathode,,a control grid a'nd a third electrode,

a source of picture signals and synchronizing signals, means for applying said picture signals to said grid to amplitude modulate said carrier wave by. picture signals, and means for applying said synchronizing signals to said third electrode to amplitude modulate said carrier wave by synsource of positive potential for said third electrode, a unilateral-conducting device connectedv between said source and said third electrode in rials, means for applying said picture signals to said grid to amplitude modulate said carrier Wave by picture signals, and means for applying said Synchronizing signals to said third electrode to amplitude modulate said carrier wave by synchronizing signals, said last means including a source '01 positive potential forsaid third electrode, a unilateral conducting device connected betweensaid source and said third electrode in a direction to conduct current to said electrode, a capacitor and an electric discharge tube connected in series with each other, said electric discharge tube having a control electrode, said series combination being connected across said device whereby said electric discharge tube connects the voltage appearing across said capacitor in series-aiding relation with said source of potential in response to the application of said synchronizing pulses to said, control electrode with positive polarity to increase the positive p0tenchronizing signals, .said last means including a so tial on said third electrode of the amplifier tube during the occurrence of a synchronizing pulse.

WALDEMAR J. POCH, 

